Block fillers are high solids coatings used on concrete masonry units (CMU's) to fill and even out the normally pitted surface. The type of block filler to use depends mostly on:

1. The conditions of service For example, an interior block wall in an apartment building likely would only require a water based, PVA or PVA /acrylic block filler. This would be particularly so if this wall is not exposed to migrating water, high humidity, or chemical attack, and is going to be finished with a conventional latex or alkyd paint.
   
   
2. The type of finish coating With some coatings it can be very important to be sure of the compatibility of the filler.
   
   
3. Type of surface All blocks are not equal. Hollow, non-load-bearing blocks can be the most porous. These are generally called lightweight blocks.
   

Desirable properties:

High solids	Filling and bridging properties. Low shrinkage
Viscosity	The material should not be overly thick as it will be difficult to apply by conventional methods. Conversely excessively thin material will not build the required thickness, or fill efficiently.
Binder solids	For the best physical properties, the binder solids-by-volume should almost equal the pigment solids-by-volume. This will give the filler more cohesive or inner film strength.
Alkali	The filler should be alkali-resistant as the 'CMU's and the mortar that holds them together are quite alkaline. High humidity or water seepage can break down non alkali-resistant fillers.
Compatibility	The filler should be compatible with the finish coating. It should supply a good base for adhesion.
Water	The filler should have enough binder (polymer) to remain cohesive, even Resistance under moderate water-saturated conditions. Without this, finish coatings with low moisture vapor permeability can show blistering and 'shearing off,' or large-scale peeling.